(1) Field of the Invention
The present invention relates to a driving apparatus for driving an imaging device, and in particular to a driving apparatus for controlling blooming, even when charges generated in each of a predetermined number of individual pixels are added together and output.
(2) Description of the Related Art
CCD (charge coupling device) image sensors have come into wide use in recent years as imaging devices in digital video cameras and digital still cameras. A CCD image sensor uses a CCD to output charges generated in each pixel. In order to improve transfer efficiency, the CCD is divided into a plurality of areas, and each area transfers the charges of a different pixel.
Transferring a plurality of charges at once in this way causes a phenomenon called blooming to occur under certain conditions, and this phenomenon prevents normal image data from being obtained.
Blooming occurs when charges that exceeds the transfer ability of the CCD are read. Charges overflow in the CCD, and affects neighboring signal charges that are being transferred. Blooming is particularly likely to occur when strong spot light is incident on the imaging device.
One means for controlling blooming is a structure in which a saturation amount is set in a range that is within the transfer ability of the CCD, and excessive charges that exceed the saturation amount is drained. This structure is called an overflow drain.
This kind of blooming control is further improved by a technique that changes the saturation amount depending on the situation, rather than setting the saturation amount to a fixed amount (see Japanese patent application publication No. S61-26375).
Meanwhile, an advancement made in recent years in the field of digital video cameras and digital still cameras is an increase in the number of pixels in imaging devices. Taking an example of a CCD image sensor that has 5,000,000 pixels, the number of vertical pixels is approximately 1920 and the number of horizontal pixels is approximately 2560. This is approximately sixteen times the number of an ordinary NTSC CCD imaging sensor. Such an increased number of pixels means that the amount of information to be processed per unit of time also increases. As a result, when taking moving images, information cannot always be processed in time before the moving image is displayed on the monitor.
One way devised to deal with this problem is a combination mode in which the charges generated in each of a predetermined number of the pixels are read and added together, and the resultant charges are then output. In a digital video camera, for example, an individual mode in which the charges from each pixel are output individually is used when taking still images, and the combination mode is used when taking moving images.
However, since the charges of a predetermined number of pixels are added together to generate one signal charge in combination mode, the problem of blooming is likely to arise if the method used to drive the CCD image sensor in combination mode is the same as that used in individual mode.